The innate immune response to pathogens is critical for the control of infection prior to the development of adaptive immunity. NKT cells with an invariant T cell antigen receptor (TCR) 1 chain (iNKT cells) are a unique T lymphocyte subset. They have been implicated in the responses to several bacteria that have glycolipid antigens that can engage their canonical TCR. iNKT cells also have been implicated, however, in the response to diverse microbes, including viruses, which do not encode antigens for this TCR. In our preliminary studies, we have found that iNKT cells are activated in vitro to produce cytokines after culture with dendritic cells (DC) infected with mouse cytomegalovirus (MCMV, a 2-herpesvirus), and this requires TLR9, IL-12 and a particular DC subset. NK cells are critical for control of acute MCMV infection, and activated iNKT cells are known to activate NK cells. The mechanism(s) governing iNKT cell activation and their contribution to the innate control of viral infection are largely unknown. Given the tools that the laboratories of the two principal investigators have assembled, we consider MCMV infection to be an ideal system for exploring how iNKT cells are activated by DNA viruses. We will decipher the mechanism(s) required for activation of iNKT cells upon MCMV infection of DC by 1) Determining the in vivo roles of TLR9, IL-12 and CD1d-mediated antigen presentation for their activation, 2) identifying the relevant DC type(s) that activate iNKT cells after MCMV infection and 3) delineating how the relevant DC subtype stimulates iNKT cells when infected with MCMV. This exploratory proposal provides a framework for uncovering the complex molecular and cellular interplay that occurs between a viral-pathogen, DC subsets and iNKT cells, which are likely to be important in antiviral immune defense. The results obtained form these studies therefore should help in devising strategies for augmenting the response to pathogenic viruses, especially members of the herpesvirus family. PUBLIC HEALTH RELAVANCE Natural killer T (NKT) cells are a unique subset of white blood cells that help to control bacterial and viral infections. However, few specifics are known about how they fight virus infection. In this project we will study how the NKT cell and other immune cells interact to mount good antiviral defenses, using a virus that is a member of the herpesvirus family. This virus is a significant cause of infant mortality, birth defects, and disease in immune suppressed individuals, such as those with AIDS.